Measuring crack growth by acoustic emission

ABSTRACT

A method and an apparatus for detecting and measuring cracks in plate-like structures using acoustic emission technique are disclosed. A false aperture transducer is designed to provide a criterion for filtering out extraneous noise in the acoustic emission signal based on modal analysis by computing the ratio of the high-frequency peak amplitude to low-frequency peak amplitude of the signal. A calibration curve correlating crack depth to the amplitude ratio can be obtained by simulating crack growth in a fracture specimen coupled to a test structure or field structure, and measuring acoustic emission signal in the structure by the false aperture transducer. The calibration curve correlates simulated crack depth percentage with computed peak amplitude ratio of the measured signal. Using the calibration curve and acoustic emission signal sensed by a false aperture transducer in a field structure, a crack in the structure can be detected and its depth measured by computing the peak amplitude ratio of the signal and identifying the crack depth that correlates with the ratio from the calibration curve.

RELATED APPLICATION

This application is a continuation of U.S. patent application. No.09/016,050, now U.S. Pat. No. 6,062,083, filed Jan. 29, 1998, which is adivisional of U.S. Pat. application Ser. No. 08/641.374, now U.S. Pat.No. 5,714,687, filed Apr. 30, 1996. Pursuant to 35 U.S.C. § 110(e),application Ser. No. 08/641,374 claims priority from the provisionalapplication, serial no. 60/007121, filed on Oct. 31, 1995.

FIELD OF THE INVENTION

This invention relates to acoustic emission testing of materials andstructures and more particularly to detection and measurement of crackgrowth in noisy environments.

BACKGROUND OF THE INVENTION

Acoustic emission is the term given to stress waves created in materialsby the sudden release of energy resulting from irreversible processessuch as crack growth, plastic deformation, and phase transformations insolid materials. Acoustic emission (AE) techniques have been used in thefield for over 25 years for the nondestructive (NDE) testing ofstructures, including metal and composite pressure vessels and piping.It has also found wide application in the testing of composite man-liftbooms. Currently, AE techniques are used primarily for locating cracksand potential problem areas in metal structures for pressure boundaryapplications, while other types of nondestructive techniques arenecessary to provide acceptance or rejection criteria. The AE technologyhas not achieved the same level of acceptance as other nondestructivetechniques for the field testing of structures such as bridges and othercomponents of infrastructures for two primary reasons: (1) thedifficulty in separating valid signals from those caused by extraneousnoise, and (2) the inability of the AE techniques to determine the sizeof the crack or flaw.

When a crack grows in a metallic material under stress due to fatigue,stress corrosion cracking, or hydrogen embrittlement cracking, a smallamount of strain energy is released in the form of a stress wave thatpropagates from the source of the crack at the velocity of sound in thematerial. The increment of time in which the strain release occurs is onthe order of a microsecond or less. Thus the frequency content of thestress wave is very broad band, ranging from a few kilocycles to over1megacycle in frequency. In solids, two types of stress waves can existin the bulk of the material: an extension wave, where the particlemotion is parallel to the direction of propagation, and a shear wave,where the particle motion is perpendicular to the direction ofpropagation.

The energy of the stress wave from the crack usually consists of both anextension wave and a shear wave. Most structures constructed from metalare plate-like in nature, examples of which include pressure vessels,bridges, and aircraft. Therefore, the stress wave shortly afterinitiation will strike a boundary. If it strikes the boundary at anangle, Snells Law prevails and mode conversion of an extensional wave toa shear wave can occur, while a shear wave can mode-convert to anextension wave. The behavior of the stress wave can become verycomplicated by the time it has traveled a distance of several platethicknesses away from the crack. In this situation, the propagatingwaves will be governed by Lamb's homogeneous equation, the solution towhich are known as Lamb waves. In the limit where the wavelength is muchlarger than the plate thickness, a simpler set of governing equationsderived from classical plate theory can be used to model the motion.Under classical plate theory, the waves are called plate waves, andthere are two modes of propagation, namely, the extensional mode and theflexural mode. Both have in-plane IP motion and out-of-plane (OOP)motion. The OOP motion is the greatest for initial displacements of asource perpendicular to the plane of the plate, and the (IP) motion isgreatest for initial displacements of a source parallel to the plane ofthe plate. For example, the sudden propagation of a crack will createprimarily an IP wave, because the crack normally grows in a directionperpendicular to the plane of the plate, while impacts on the surface ofthe plate will create primarily OOP sources, since the initial sourcefunction creates a bending or flexural wave.

The stress waves (acoustic emission (AE) events) in present practice aredetected by a piezoelectric transducer that is attached to the surfaceof the structure with vacuum grease, vaseline, or other couplants toprovide an air-free path for the high frequency waves to reach theactive element of the transducer. The transducer used in the majority ofthe tests has a resonant frequency of approximately 150 kHz. When the AEwaves strike the transducer, they set it “ringing” at its resonantfrequency. The use of a resonant transducer increases the sensitivity ofdetecting the AE, events. Since the frequency contents of the waves arevery broad band, they will activate the resonant frequency of anytransducer having a resonant frequency between 20 kHz and 1 MHz. Most AEdata is taken in the 100 to 500 kHz frequency range, where the data islow enough in frequency that attenuation effects are minimal, and highenough in frequency so that low frequency air borne noise is eliminated.

A majority of the practical AE tests are conducted on structures madefrom plates or plate-like components. Recent research has shown thatout-of-plane (OOP) Al sources produce strong flexural wave components ina plate, with weak extensional components, while in-plane (IP) AEsources produce strong extensional waves in a plate with weakdisplacement components normal to the plate surface.

For many years, researchers and field test engineers employing acousticemission techniques have used the breaking of a pencil lead on thesurface of a structure or specimen to simulate the type of AE signalpresent when a crack propagates or when fibers break in compositestructures. Because this is an OOP source, most of the energy goes intothe flexural wave which is inherently low frequency, and only a smallportion of the energy is carried by an extensional wave.

Michael R. Gorman, in his paper entitled “Plate Wave Acoustic Emission,”Journal of Acoustic Society of America, 90(1), July 1991, used broadband sensors to detect both types of waves. By mounting the transduceron the surface of an aluminum plate, extensional waves were simulated bybreaking pencil leads on the edge of the plate, and flexural waves weresimulated by breaking the pencil lead on the surface of the plate.Further work by Gorman and Prosser, “AE Source Orientation by Plate WaveAnalysis,” Journal of Acoustic Emission, Vol. 9, No. 4 (1990), consistedof machining slots at different angles in a plate to observe theresponse when pencil leads are broken at an angle, which is measuredfrom the plane of the plate. As expected, it was found that for 0degrees, the highest signal amplitudes occurred for extensional waves,and for 90 degrees, the highest signal amplitudes occurred for flexuralwaves. A mixture of both waves was found for intermediate angles.

The broad band transducer used by Gorman and Prosser is problematic fora number of reasons. First, it was designed for ultrasonic testing witha resonant frequency of 3.5 MHz, and was presumed to have a flatfrequency response from a few kHz to 1 MHz. However, although themeasured frequency response of similar transducers yields a fairly flatfrequency response from 300 kHz to 1 MHz, it is far from flat from 10kHz to 300 kHz. In addition, its sensitivity in the frequency rangebelow 1 MHz is an order of magnitude lower than those of resonanttransducers normally used for acoustic emission testing. Consequently,the sensor must be placed close to the source in order to obtain goodresults. Moreover, when IP and OOP amplitudes are compared from dataobtained by the transducer mounted on the surface, there is a largedifference in the peak amplitudes measured from the different sources.Further, the procedure presently employed by Gorman cannot be used forcrack growth measurement. Gorman merely discloses a method to digitizeall signals and to attempt through visual examination and patternrecognition software to determine the amount of IP and OOP componentspresent to make a decision regarding whether or not a signal isprimarily one or the other type. Because the overwhelming number of AEsignals detected in the field are extraneous noise (OOP), Gorman'smethod requires an enormous amount of storage for the digitized signals.

One of the major reasons AE techniques have not been widely accepted isthat the techniques will not give any quantitative informationconcerning the size of a crack or the amount of crack growth. Currently,AE is primarily used to locate the crack by the use of multiplechannels. By measuring the time that each transducer receives the AEsignal, and ascertaining the velocity of sound in the material, thesource location can be calculated.

The second main reason for the lack of wide application of AE techniquesto monitor structures in the field is the difficulty in separatingextraneous background noise from the AE signals coming from the crack.Impacts on the field structure from wind-blown sand, particles, rain,maintenance personnel, and leaks in pressurized components all can givenoise signals in the frequency band of interest. Rubbing frictionbetween components is another source of extraneous noise that hasfrequency components in the frequency range of interest. Most extraneousnoise sources of this type are out-of-plane (OOP) sources, and althoughthey can have very high-frequency components in an undamped structure,most of the energy in the stress waves created by such sources in moststructures constructed from plates can be found at frequencies below 100kHz. This energy is carried by a low-frequency flexural wave in theplate. AE signals generated by crack growth, on the other hand, arein-plane (IP) sources, and most of the energy in the stress wave iscarried by high-frequency extensional and shear waves.

SUMMARY OF THE INVENTION

There is a need, therefore, for a method and an apparatus of eliminatingextraneous noise in an AE signal and determining crack size from the AEsignal.

This invention provides a solution to the above major problemsencountered in current acoustic emission testing by using the ratio ofthe extensional wave to the flexural wave as a filter for eliminatingextraneous noise and determining crack size. Special transducer andinstrumentation techniques are used to recognize the type of wavepredominant in a plate or plate-like structure to allow filters to beconstructed in the instrumentation to not only eliminate extraneousnoise sources from the AE data, but also make available nondestructivemeasurement of the depth of a growing crack in the structure by AEtechniques. In addition, OOP signals are eliminated early on so that asmaller amount of storage is required to store the data for crack-likeIP signals. The invention will be discussed in relation to crack growthin metals, but is not restricted to metals only.

In accordance with one aspect of the present invention, a false aperturetransducer is formed by partially mass-loading a large crystal with amass. The partially mass-loaded portion of the crystal reacts as adisplacement-sensitive element and responds well to out-of-plane (OOP)sources, while the remainder of the crystal is sensitive to the higherfrequency in-plane (IP) sources. The ratio of sensitivity to thehigh-frequency IP source and low-frequency OOP source can be calibratedto a specific value. The presence of an OOP signal will produce a ratioof the high-to-low frequency amplitude of lower than the sensitivityratio of the transducer and an IP signal will produce a frequencyamplitude ratio higher than the sensitivity ratio. In a preferredembodiment, the sensitivity ratio of the transducer is calibrated to beequal to one so that the transducer is equally sensitive to OOP and IPsources.

To obtain the high-to-low frequency amplitude ratio of an Al signal, afalse aperture transducer is used to sense the signal. The AE signal issplit into a high-frequency component and low-frequency component withanalog filters and amplified. The high-frequency component is filteredwith a high-pass filter, and the low-frequency component is filteredwith a bandpass filter. The peak amplitudes of the two components aremeasured, from which the frequency amplitude ratio is calculated.

The high-to-low frequency amplitude ratio can be used to filter outextraneous noise by eliminating ratios that are lower than thecalibrated sensitivity ratio of IP to OOP sources of the false aperturetransducer. A portable instrument can produce an audio signal to anoperator upon detecting a ratio higher than the sensitivity ratio, whichindicates a crack-like IP source. Location of the crack-like source canbe determined by detecting the AE signal from three different locationsand ascertaining the point of intersection.

Significantly, the determination of crack size is also made possible bycomputing the high-to-low frequency amplitude ratio. To do so, acalibration curve correlating crack depth with the frequency amplituderatio is obtained by cloning a fracture specimen or similar block ofmaterial onto the structure to be tested or a plate-like specimen withsimilar thickness. Crack growth is simulated in the fracture specimenwhich is attached to the structure. A false aperture transducer on thestructure receives the AE signal generated and the high-to-low frequencyamplitude ratio is computed. By simulating different crack depths in thefracture specimen and computing corresponding amplitude ratios from theAE signal, a calibration curve is obtained. To measure crack depth, thefalse aperture transducer is placed on the structure and AE signals sentto an instrumentation for computing the high-to-low frequency amplituderatio. The corresponding crack depth can be determined with theamplitude ratio by the calibration curve. The crack profile isapproximated by assuming a semi-elliptical shape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front elevational view of a transducer illustrating apreferred embodiment of the present invention.

FIG. 2 is a plot illustrating the response of the transducer of FIG. 1to out-of-plane and in-plane acoustic emission signals.

FIG. 3 is an apparatus for experimentally designing a transducer fordesirable sensitivity to in-plane and out-of-plane signals.

FIG. 4 is a schematic view of a circuit for filtering extraneous noisein an acoustic emission signal.

FIG. 5 is a block diagram of an instrumentation for locating acrack-like source in a structure illustrating another preferredembodiment of the present invention.

FIG. 6 is a graph of peak detected signals from the transducer of FIG. 1obtained with the instrumentation of FIG. 5.

FIG. 7 is a block diagram of a crack measurement instrumentationillustrating another preferred embodiment of the present invention.

FIG. 8 is an apparatus for calibrating the instrumentation of FIG. 7.

FIG. 9 is a graph showing the low-frequency data from the specimentransducer of FIG. 8.

FIG. 10 is a calibration curve obtained with the apparatus of FIG. 8 tocorrelate frequency peak amplitude ratio with crack depth and toseparate in-plane from out-of-plane signals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

To filter out extraneous noise and measure crack size, a transducer 10shown in FIG. 1 is employed. The transducer 10 responds to in-plane andout-of-plane acoustic emission signals with a particular sensitivity.Through modal analysis of the high-frequency and low-frequencycomponents of the signal sensed by the transducer 10, a filter can beconstructed to filter out extraneous noise, an instrumentation can beconfigured to locate a crack and measure the size of the crack.

A. Dual Purpose False Aperture Transducer

Most acoustic emission transducers respond to both OOP and IP sourcesand detect the presence of both modes for either of the above inputs.For filtering out noise and ease of analysis, a desirable transducerwould respond to an IP source with no evidence of OOP signals and to OOPsources with no evidence of IP signals. Such a transducer can beconstructed by a combination of a small aperture mass-loaded componentand a larger aperture nonmass-loaded component. One embodiment is alarge aperture nonmass-loaded transducer which includes a “false” smallaperture created by partially mass-loading a larger crystal element.This partially mass-loaded portion of the crystal would react as adisplacement-sensitive element and respond well to OOP sources, whilethe remainder of the crystal element would be sensitive to the higherfrequency IP sources.

A preferred embodiment of the present invention is a transducer 10comprising a piezoelectric crystal 12 that is partially mass-loaded by amass 16 in the center, as illustrated in FIG. 1. The size of the crystal12 and the size and size of the mass 16 are preferably selected toprovide approximately equal amplitude signals for a pure OOP signal anda pure IP signal. This hybrid design advantageously achieves a balancein detection of signals, making the transducer 10 equally sensitive toboth OOP and IP signals. Significantly, this facilitates the analysis ofthe measured data by simply examining the amplitude ratio of the twosignals, as discussed below.

Referring to FIG. 1, the piezoelectric element 12 is desirably enclosedby a metal case 18. The transducer 10 is shown coupled to a structure tobe tested, plate 20, with a couplant, desirably vaseline. A ceramic disk24 advantageously provides electrical isolation between thepiezoelectric element 12 and the plate 20. The top surface 26 of theceramic disk 24 is desirably coated with a conductive epoxy in order tomake electrical contact between the bottom 28 of the piezoelectricelement 12 and the metal case 18. The piezoelectric element 12 in thisembodiment is desirably a circular cylindrical member, desirably ofabout 0.5 inch and more desirably 0.5 inch in diameter, desirably about0.15 inch and more desirably 0.150 inch in thickness, and is preferablypolarized in the thickness expander mode. A hole 30 of about 0.1 inch,preferably 0.100 inch, in the center through the top surface 36 of thecrystal 12 helps to broaden the frequency response. The mass 16 isdesirably a ball bearing, preferably made of a tungsten carbide, whichis desirably bonded to the center of the crystal 12 and partially coversthe top surface 36 of the crystal. The ball bearing is desirably about0.25 inch, more desirably 0.250 inch, in diameter, and about 0.125 inch,more desirably 0.125 inch, flat. This partial mass-loading creates a“false” aperture and makes the piezoelectric element 12 react in adisplacement mode for this portion of the crystal 12. The transducer 10is thus referred to as the false aperture transducer 10.

Breaking pencil leads on the surface of the plate 20 creates a flexurewave in the plate 20 which is low frequency in nature and gives largedisplacements as the wave travels past the transducer 10. Breakingpencil leads on the end of the plate 20 creates extension waves in theplate 20 with only very small displacements occurring at the surface ofthe plate 20 as the wave travels past the transducer 10. Modeconversions of both an extensional wave and a shear wave occur forfrequency components of the AE pulse that have wavelengths smaller thanthe thickness of the crystal element 12. These higher frequency wavespropagate into the crystal 12 and set it into resonance in areas notloaded by the mass 16. When a displacement of one of the top surface 36or the bottom surface 28 of the crystal 12 occurs, an equal and oppositeelectrical charge appears on the each of the top and bottom surfaces ofthe crystal element 12. This electrical signal is transmitted through aconnector 40 for input into other instrumentation (not shown). Theinside of the case 18 is desirably filled with a polyurethane rubbercompound (not shown) in order to dampen resonance of the ball bearing 16and to provide protection against shock.

FIGS. 2 a and 2 b show the response of the false aperture transducer 10to OOP and IP waves in a ½ inch thick steel plate of 48 inches in lengthwith pencil lead breaks made at one end and the transducer 10 located at24 inches from the end. FIG. 2 a shows the response to an OOP signal,which is the type of signal that would be observed for extraneous noise,such as impact on the bar, rubbing friction, and airborne noise. FIG. 2b shows the signal from an IP source which may be created by breaking apencil lead on the end near the mid-plane of the plate. This simulatesthe type of signal that is created by crack growth. Dividing theamplitude of the IP signal by the amplitude of the OOP signal gives aratio of less than 1. Experiments have shown that the presence of an OOPsignal would produce a ratio of the high-to-low frequency amplitude ofmuch less than 1, whereas an IP signal would give a ratio of high-to-lowfrequency of greater than 1. As discussed below, this frequency peakamplitude ratio can be used not only to design a filter to eliminateextraneous noise, but to compute crack growth.

Other transducers can be configured to possess the characteristics ofthe dual component system exhibited in the false aperture transducer 10of FIG. 1 just described. Experiments have shown that a small aperturemass-loaded transducer is the best type of transducer to use foraccurately defining the displacement and frequency content of OOPsources. This type of transducer is very insensitive to IP sources whichone associates with crack growth. On the other hand, a large aperturetransducer without mass loading is very sensitive to IP signals and lesssensitive to OOP signals. For equal sensitivity to IP and OOP signals, adesirable transducer would have a small aperture mass-loaded elementwith a larger nonmass-loaded element, which may be achieved by having alarge piezoelectric element with a hole in the center to place thesmaller mass-loaded element. The same effect can be accomplished bysimply placing a small mass at the center of the large aperture element.By adjusting (1) the size of the mass, (2) the relative area of thelarge aperture element covered by the mass, (3) the size (diameter andthickness) of the large aperture element, (4) the aperture size, and (5)the diameter of the ceramic disk, the sensitivity of the transducer toOOP and IP sources of AE signals can be adjusted so that it has equalsensitivity for both types of signals or a desirable ratio ofsensitivity.

To illustrate a method of selecting a set of desirable parameters for atransducer, the experimental procedure for obtaining the parameters forthe preferred embodiment of the transducer 10 of FIG. 1 is described. Itis understood that this is an example to illustrate, not to restrict,the methodology. The procedure involves testing transducers havingsimilar configuration as the transducer 10 but with different aperturesizes and mass loading for detecting plate waves. For instance, twonarrow steel plates of two different thicknesses are subjected to bothout-of-plane (OOP) and in-plane (IP) sources and the response oftransducers having several different aperture sizes mounted on theplates are observed. The OOP source can be simulated by the breaking ofa 0.3 mm 2H pencil lead on the surface at one end of the plate, and theIP source from breaking leads on the end of the plate. The transducer isdesirably placed midway on the surface of the plate for both situations.Different aperture sizes can be created desirably by bonding 0.040 inchthick ceramic disks of different diameters to a crystal element in thetransducer. Epoxy is desirably cast around the disks and then machinedto expose the ceramic disk and provide a supporting rim to keep correctalignment during the testing. Two mild steel metal plates 48 inches longby 2 inches wide are used.

FIG. 3 shows a schematic of the apparatus for carrying the experimentalprocedure. A plate 60 is used to simulate a test structure, one ¼ inchthick and the other ½ inch thick. The plate 60 is coated on the bottomsurface with a damping material such as a rubber compound 62 to moreclosely simulate actual structures in the field. For instance, pressurevessels and piping normally have something inside that is effective inabsorbing acoustic energy propagating in the wall, and most otherstructures have coats of paint or other materials on the surface thatact to absorb energy from the plate waves generated by a crack or leak.The damped plates are designed to simulate these conditions.

A trigger transducer 66 generates a trigger signal and starts the sweepof a transient recorder 68. A data transducer 70 is coupled to the plate60 and receives AE signal which is pre-amplified by a pre-amplifier 72.The signal is then split into high-frequency and low-frequencycomponents that are filtered by filters 74. The components then passthrough the transient recorder 68 and the result is sent to a plotter78.

Most of the data is taken with the acoustic emission transducer placedmidway along the length of the plate 60 (24 inches). A 45 degree flatbottomed hole 80 is desirably machined in one end of each plate 60. Thisprovides the capability of breaking pencil leads at a 45 degree angle ordirectly on the flat portion of the hole 80 to produce an in-plane (IP)signal in the plate 60. Out-of-plane signals can be generated bybreaking leads on the top of the plate 60 at one end. This setup shownin FIG. 3 may be modified for some of the experiments by positioning a ⅛inch aperture 650 kllz transducer (not shown) at the end where thepencil leads are broken. This provides a zero time reference to the dataand allows velocity measurements to be made on the various types ofwaves produced. The transient recorder 68 is desirably an A/D converterwith a sampling rate of 20 MHz and 8 bits of resolution.

After trying several combinations of different crystal elements andmasses, the transducer 10 of FIG. 1 is selected, having a 0.500-inchdiameter PZT element of 0.150 inches in thickness with a 0.100-inch holein the center as the large aperture element, and a 0.250-tungstencarbide ball bearing with an ⅛-inch diameter flat bonded in the centerof as the small mass-loaded in the center of the larger apertureelement. When the transducer 10 of this type is placed on the surface ofthe plate 20 and a wave from an OOP source travels down the plate 20 ofFIG. 1, the mass-loaded center of the crystal 12 acts as a displacementdevice and gives a large amplitude signal from this OOP source. When anextensional wave is present in the plate 20 from an IP source, verylittle displacement occurs at the surface, but mode conversions from theextensional wave occurs and starts the larger aperture element 12 to“ring” at its resonant frequency. A signal for these types of wavestraveling at the shear velocity in the material is also observed.

Although a transducer that is equally sensitive to both OOP and IPsignals is desirable in practice, as discussed in its application below,it may be advantageous for a transducer to be more sensitive to one ofthe two types of signals in some situations. For instance, a smallspecimen produces a lot of high frequency signals from reflection withinthe boundary of the specimen during testing. A transducer that is moresensitive to OOP signals than to IP signals would be more appropriateand effective for acquiring data in a small specimen. An example is atransducer that has an enhanced response to OOP signals, comprising apiezoelectric element of ⅛ inch in diameter and 0.05 inch in thicknessmass-loaded by a tungsten carbide ball bearing of ¼ inch in diameter andhaving an ⅛ inch flat.

As long as the ratio of the sensitivity of a transducer to OOP and IPsignals is known, the transducer can be used to filter out extraneousnoise and measure crack growth via modal analysis, as discussed indetail below. The construction of an AE transducer can thus be tailoredto the specific structure and application. It is thus contemplated thata transducer that is configured and calibrated for a specificsensitivity to OOP and IP signals in acoustic emission testing wouldfall within the scope of this invention.

B. Filtering Out Extraneous Noise

The first application of the modal analysis of the OOP and IP signals isfiltering out extraneous noise. Noise in an acoustic emission test, suchas impact of particles due to rain and dust, and noise due to friction,are primarily OOP sources. AE signals due to crack growth in metals andother materials, and fiber breakage in composites, are IP sources. Therequirement that an AE signal be an in-plane (IP) signal is the firstcriteria for acceptance of the AE data. Eliminating the noise early inthe data gathering process will make analysis of crack-like signals mucheasier. The only IP signal that could be considered extraneous noisewould be due to crushing of oxides on the crack surface at minimum loadsfrom a crack undergoing cyclic loading. Furthermore, signals havingstrong OOP components could be eliminated early in the data gatheringprocess in order to eliminate the mass storage problems and dataanalysis of hundreds of thousands of signals in some cases.

If a transducer can recognize which type of source is present, a frontend filter can be set up, such that only signals that can be recognizedas IP sources are accepted. For instance, one may split the AE signalfrom the transducer of FIG. 1 into high and low frequency bands, subjectthem to a 20-60 kHz (or 20-80 kHz) bandpass filter and a 100 kHzhigh-pass filter, measure their peak amplitudes respectively, and thendivide the high-frequency signal by the low-frequency signal. If theratio is greater than 1, the high-frequency signal is accepted as comingfrom crack growth. The ratio for acceptance can be set based ondesigning a plate having the same thickness of the structure to betested.

An embodiment of a circuit for carrying out the filtering function isillustrated in FIG. 4. As shown in FIG. 4, a multiple-stage, activefiltering circuit includes a plurality of operational amplifiers U1-U10.Each of the operational amplifiers U1-U3 advantageously comprises OP37operational amplifiers. U4 advantageously comprises an OP27 operationalamplifier. Each of the operational amplifiers U5-U8 advantageouslycomprises TL071 or equivalent operational amplifiers. U9 and U10 eachdesirably comprises an LM319 or equivalent operational amplifier. Eachof the operational amplifiers U1-U10 is powered using a ±15 volt source.The values of the various circuit components for one advantageousembodiment of the filtering circuit are as depicted in FIG. 4, althoughit will be appreciated by one of ordinary skill in the art that avariety of other particular circuit configurations might be used toimplement the filtering operation provided by the circuit of FIG. 4.

In operation, the circuit of FIG. 4 takes the signals from the falseaperture transducer at an input terminal 82, amplifies it by 40 dB, andsplits it into 2 frequency bands which are output on terminals 84, 86.The low frequency band spans 20-60 kHz, the high band spans 100 kHz toseveral MHz. The output of each frequency band is full-wave rectifiedand sent to a track-and-hold circuit which tracks the rectifier outputand holds the peak value. The track-and-hold output goes to a thresholddetector with an adjustable setting. A logic high is generated if thethreshold is crossed. A reset input allows the track-and-hold circuitsto be reset.

The output of the false aperture transducer is capacitively coupled tothe amplifier U1 (together with its associated bias circuitry) having again of approximately 10. The output of U1 feeds two channels: thelow-frequency channel above and the high-frequency channel below.

The low-frequency channel is made up of a 20 kHz 2-pole high-passfilter, followed by a 60 kHz 2-pole low-pass filter. Thus, a netbandpass filter is created by the cooperation of the low and high passfilters. The high-pass filter comprises U2 and associated components,while the low-pass filter comprises U5 and associated components. In onedesirably embodiment, the high-pass filter has a gain of 10, while thelow-pass filter a gain of 1.

The high-frequency channel is defined as a 100 kHz, 2-pole, high-passfilter with a gain of 10. The high-pass filter used to define thehigh-frequency channel comprises the operational amplifier U3 and itsassociated components.

The rectifiers, track-and-hold circuits, and threshold detectors for thetwo channels are identical. In particular, the rectifier andtrack-and-hold circuit for the low frequency channel are shown to beenclosed in dashed lines as the circuit 90 (including inverter U7 andbuffer U8), while the rectifier and track-and-hold circuit for the highfrequency channel are shown to be enclosed in dashed lines as thecircuit 92 (including inverter U4 and buffer U6). In like manner, thethreshold detector for the low-frequency channel is shown to be enclosedwithin dashed lines and designated by the reference numeral 94(including differential amplifier U9), while the threshold detector forthe high-frequency channel is shown to be enclosed within dashed linesand designated by the reference numeral 96 (including differentialamplifier U10).

The basic track-and-hold circuit is configured as an emitter followedwith a capacitor, instead of a resistor, as its load. The emitterfollower can charge the capacitor, but not discharge it, so the emitterfollower circuit acts like an electrical ratchet (i.e., where energy isinput but not released). The voltage on the capacitor follows the inputvoltage up and holds the peak value. To make this a full-wave circuit sothat it follows both positive and negative peaks, two emitter followingtransistors are used (shown in a “face-to-face” configuration), bothconnected to the same capacitor. The input of the second emitterfollower is the output of a unity gain inverter. In the low-frequencychannel, the inverter is U7, and in the high-frequency channel, it isU4.

Each of the two hold capacitors is shunted by an NPN transistor whichdischarges its associated capacitor when turned on by a reset signal.The reset signal is a logic high provided externally. That signal turnson a grounded-base PNP transistor 98 that turns on NPN transistors 102,104. The 10 k resistors in the emitter follower base and collectorcircuits limit the currents to safe values when the NPNs are on. Thecollector resistors are bypassed by 0.1uF capacitors to provide thebrief surges of high current required to track the signal. In anadvantageous embodiment, all NPN transistors are 2N3904 transistors andthe PNP transistor is either a 2N3906 or 2N4403 transistor.

The output of each track-and-hold circuit is buffered by a unity gainamplifier with a diode in its feedback to compensate for the Vbe offsetof the track-and-hold circuit. The buffer is U8 for the low-frequencychannel and U6 for the highfrequency channel. The buffer outputs arebrought out so they can be digitized by external equipment, and also areinput to a pair of threshold detectors.

The threshold detector for the low-frequency channel comprises U9, whilethe threshold detector for the high-frequency channel comprises U10.Thus, a dual comparator configuration is defined. The reference input ofthe detectors are potentiometers connected across the positive supply sothat the threshold can be set by the user. The comparator outputsprovide a logic high when the threshold is crossed. With propersoftware, the user can define an event by either of the two thresholdcrossings, or by both.

The circuit 106 (shown at the bottom of FIG. 4) provides TTL outputs toa counter to provide “ring-down” counts for data analysis. In oneembodiment, the outputs of this circuit are designed to interface with aPC-LPM-16 data acquisition board from National Instruments (not shown).Two channels of A/D (for the low and high frequency peak amplitudevalues), two channels of digital input (for the low and high frequencythreshold crossings), one digital output (for reset), one timer (toestablish a delay between sensing a threshold crossing and reading thepeaks) and one counter (to accumulate threshold crossing counts)comprise the data stream for the circuit and DAQ board (not shown).

For filtering purposes, software for the computer calculates thehigh-to-low frequency amplitude ratio, and compares this ratio to avalue input by the user (this value established by tests on a plate ofcomparable thickness to show the ratio of OOP to IP sources). If thevoltage ratio exceeds this value, the ring-down counts will be acceptedfrom the high frequency channel. If the voltage ratio is lower than thisvalue, the AE counts data is not accepted, since the signal will be anOOP signal, which comprises extraneous noise.

C. Locating a Crack-Like Source

The transducer 10 of FIG. 1 can be used to locate a crack-like source. Apreferred embodiment of an instrument incorporating the transducer 10 isa unit that is desirably portable, making it suitable for bridge testingand other field measurements. FIG. 5 is a block diagram of theinstrument. The block diagram indicates how the difference in extraneousnoise and crack-like signals can be determined with a portable systemand audio earphones.

Referring to FIG. 5, the AE signal is detected by a transducer 110 andthe signal is split into the high frequency channel 114 and lowfrequency channel 116 shown. An amplifier 120 amplifies thehigh-frequency signal, which then passes through a high-pass filter 128which is desirably a 100 kHz filter. The low-frequency signal isamplified by the amplifier 122 and filtered by a band pass filter 130which is desirably 20-60 kHz. A peak detector circuit 136 determines thehigh-frequency peak amplitude/voltage of the high-frequency component,and a peak detector circuit 138 determines the low-frequency peakamplitude voltage. An alternative embodiment may replace the twoamplifiers 120 and 122 by a single amplifier (not shown) which amplifiesthe AE signal prior to splitting of the signal into high-frequency (HF)and low-frequency (LF) components. Such an embodiment is mostappropriate for the transducer 10 which is equally sensitive to OOP andIP signals. The circuit of FIG. 4 may be used to perform the steps justdescribed to obtain high-frequency and low-frequency peak amplitudes.

The low-frequency and high-frequency amplitudes are used to filter outextraneous noise and locate the crack. In one embodiment, an audiosystem is employed to indicate the presence of a crack-like signal. Asshown in FIG. 5, the high-frequency peak voltage is then applied to afirst voltage-controlled oscillator 144 which will generate a 3 secondtone dependent on the peak voltage. A second voltage-controlledoscillator 146 generates a 3-second tone from the low-frequency peakvoltage. A first audio amplifier 152 amplifies the high-frequency toneand applies it to an earphone 154 worn by the operator. A second audioamplifier 156 amplifies the low-frequency tone which is then fed toanother earphone 158. Assuming that the high-frequency channel goes tothe right earphone 154 and the low-frequency channel goes to the leftearphone 158, all the operator needs to observe is whether or not theright earphone 154 has a higher pitch than the left earphone 155. If theright earphone 154 does have a higher pitch, the signal detected is anIP signal and is more than likely coming from crack growth. To use theportable testing unit, one would carry the instrument in a batteryoperated configuration, and wear a pair of earphones plugged into theinstrument to search for AE signals that can be identified as comingfrom a crack-like source, as opposed to extraneous noise.

Alternatively, the two peak voltages from the high-frequency andlow-frequency channels may be compared to determine whether the analog,high-to-low frequency peak amplitude ratio is greater than 1 (notshown). If the ratio is greater than 1, a tone is generated andaudio-amplified. The operator can thus identify the tone as indicatingdetection of a crack.

In situations where the transducer 110 is not calibrated to be equallysensitive to OOP and IP signals, the amplification of each channel canbe varied independently by calibrating the amplifiers 120 and 122 toadjust the peak voltage in each channel such that a high-to-lowfrequency peak amplitude ratio of one or less corresponds to an OOPsignal as determined in the calibration sequence on the particularstructure under test. In this manner, the dynamic range of the tonesgenerated in each channel will be calibrated. For example, a 440 A tonemay be set to correspond to a ratio of one with a dynamic range of oneoctave higher and lower than 440 to cover a specified range of voltageratios.

If the operator determines that a crack-like source is within the rangeof the instrument, the location of the source can be found as follows.Because the velocity of the extensional wave is the highest andnondispersive, and the leading edge of the low-frequency dispersive wavearrives at the shear velocity, the time difference between the twosignals will vary as a function of the distance of the source from thetransducer 110. An example is illustrated in FIG. 6, which shows theoutput of the peak detection circuit 136 for the high-frequency channel114 and the peak detection circuit 138 for the low-frequency channel 116when the false aperture transducer 10 of FIG. 1 is placed at 12 inchesand 24 inches distance from the end of a ½ inch thick steel plate. 0.3mm pencil lead breaks are made at 25% depth at the end of the plate toproduce the signals shown in FIG. 6. The time difference between thehigh and low frequency signals represented by T₀ and T₁, in FIG. 6varies as a function of the distance of the transducer 110 from thesource. This time difference can be calculated from the known velocitiesof the waves in the plate in the following manner:

Let T_(e) equal the time required for an extensional wave to travel adistance D at a velocity V_(e), then

T_(e)=D/V_(e)  (1)

and T₃₂=D/V_(s) is the time required for a wave traveling at the shearvelocity to cover the same distance D. The difference is

T_(s)−T_(e)=D(1/V_(s)−1/V_(e))=T₀ or T₁ in FIG. 6  (2)

For steel:

V_(e)=2.13×10⁵ in/sec and V_(s)=1.23×10⁵ in/sec  (3)

From (2) and (3)

T_(s)−T_(e)=D(3.44×10⁻⁶)  (4)

D=(T_(s)−T_(e))/3.44×10⁻⁶(5)

In the example illustrated in FIG. 6, the calculated values for T₀ andT₁, (Equation 4) are:

T₀=12(3.44×10⁻⁶)=41 microseconds and

T₁=24(3.44×10⁻⁶)=83 microseconds.

The measured data in FIG. 6 correlates well with these values. Thedistance D can thus be solved in accordance with Equation (5).

Referring to FIG. 5, if the threshold detection of the high frequencychannel 114 is used to start the cycle counter 164 in counting thecycles of a 1 MHz clock 166, and the threshold detection of the lowfrequency channel 116 is used to stop the cycle counter 164, the numberof cycles counted will correspond to the number of microseconds delaybetween the extensional wave and lower frequency flexural wave. (One canthen solve for the distance 174 from the transducer to the source byEquation 4 above. This distance 174 corresponds to the radius of acircle D, and the source is located somewhere on this circle. If onemoves the transducer 110 a known distance twice and again makes the sametime measurements and calculations, the source corresponds to theintersection of the three vectors defined.

In yet another embodiment (not shown), the 1 MHz clock 166 and cyclecounter 164 are used to output the peak detected signals to a portabledigital oscilloscope (not shown) and to capture the signals and make avisual measurement of the time differences. This method can be used togenerate the signals in FIG. 6. The time difference can be used tocompute the distance D according to Equation (5).

D. Measuring Crack Size

Crack growth in a structure or specimen predominately occurs normal tothe plane of the plate carrying a tension load. The incrementalextension of the crack due to events such as fatigue, stress corrosioncracking, and hydrogen embrittlement, becomes an IP source for acousticemission signals, which can be simulated by the breaking of a pencillead on the edge of a structure at different depths along the thicknessof the structure. Signals originating from these IP sources producedifferent types of signals. There is an increasing amount of flexuralwaves for crack growth nearing the surface as compared with crack growthat mid-thickness. For example, if during a fatigue test of a specimenwith a thumbnail surface crack, one observes a decreasing amount offlexural wave component in the AE signal as the crack grows, this wouldbe indicative of a crack having a depth less than half the distancethrough the plate. On the other hand, if one observes an increasingamount of flexural component as the crack grows, this would be anindication that the crack is deeper than one-half the thickness of theplate. However, the amplitude of the high-frequency extensional waveremains unchanged. Its amplitude is independent of where the pencil leadis broken along the thickness. This is an important phenomenon and isthe key to keeping the ratios of the two frequencies independent of theabsolute amplitude of the initial detected signal.

The crack depth can be defined in more detail by correlating it with theratio of the high-to-low frequency components of the signals from atransducer in accordance with the present invention. The ratio of thehigh-frequency to low-frequency amplitudes can be correlated todetermine where in the depth of a structure that a real crack ispropagating, regardless of the amplitude of the signal created by theincremental crack growth, since the amount of flexure wave createdshould only be a function of the moment around the neutral axis and notthe amplitude. The ratio increases as a surface crack grows in depth ofa plate or plate-like structure, reaches a maximum at mid-thickness, anddecreases as the crack grows deeper.

This correlation forms a set of data that could then be used in thefield on structures constructed from plates or plate-like components ofthe same material to estimate crack depth. The calibration plates andstructures do not need to have the same thickness because thecorrelation is based on percentage depth. Cracks do not usually grow byextension across the whole crack front, especially in thicker plates.Rather, surface cracks generally grow in a semi-elliptical shape.Therefore, careful measurement of signal mode ratios (high-to-lowfrequency amplitude ratios) will allow one to determine not only themaximum depth of the crack, but also the profile or shape of the crack.

For all cracks, including through cracks, the acoustic emission countscan be used to estimate the amount of crack growth, and the voltageratios will be used primarily as a filter to eliminate extraneous OOPsources of AE signals. See H. L. Dunegan & A. S. Tetelman,“Non-Destructive Characterization of Hydrogen-Embrittlement Cracking byAcoustic Emission Techniques,” Engineering Fracture Mechanics, Vol. 2,pp. 387-402 (1971), which is hereby incorporated by reference.

An instrumentation for detecting crack size and crack growth in astructure is illustrated in the block diagram of FIG. 7. A datatransducer 210 is coupled to the structure (not shown) to be examined. Atrigger transducer 214 generates a trigger signal through an amplifier216 and starts the sweep of a digital oscilloscope or voltmeter 220. Thesignal from the data transducer 210 is split into a high-frequency (HF)component and a low-frequency (LF) component. The 100 kHz high-passfilter 226 filters the high-frequency component, and the 20-60 kHzbandpass filter 228 filters the low-frequency component. Variable gainamplifiers 234 and 236 are used to amplify the two components, which arethen input into the digital oscilloscope or voltmeter 220. The digitaloscilloscope 220 captures the transient signals and passes them to thecomputer 244, desirably through a fiber optic cable (not shown). Aprinter 248 prints the waveforms. When a signal is detected orsimulated, such as by breaking a pencil lead at a given depth, both thehigh frequency and low frequency waveforms are captured and printed.

1. Calibration

In order to predict crack depth and growth from acoustic emissionsignals, a calibration curve illustrating crack depth as a function ofthe ratio of the high-to-low frequency components of the AE signals isdesirably employed. To obtain a calibration curve, the first step is tocouple a fracture specimen or similar block of metal as mentionedpreviously to the structure to be tested.

An in-plane acoustic emission signal is then simulated in the fracturespecimen to simulate crack growth with 0.3 mm Pentil pencil leads. Thisis repeated at different depths of the specimen. The resulting acousticemission signal in the structure is detected by the false aperturetransducer. From the signal in the structure, the ratio of thehigh-to-low frequency components (HF/LF) is computed for each simulatedcrack depth. Signals from extraneous noise sources indicated by afrequency peak amplitude ratio of less than 1 is eliminated, providedthat the signal measurement of the structure is performed with atransducer calibrated for equal sensitivity to in-plane (IP) andout-of-plane (OOP) signals, such as the false aperture transducer 10 ofFIG. 1. A calibration curve is then plotted for the HF/LF ratio versuscrack depth, and is used to correlate frequency peak amplitude ratiowith crack depth and to separate IP from OOP signals.

To illustrate the calibration procedure in a laboratory, an example isgiven as follows. For application to bridges and other thick-platestructures, steel plates of ¼ and ¾-inch thickness are suitablecalibration plates. FIG. 8 shows the apparatus incorporating a ¾-inchthick compact-tension (CT) fracture toughness specimen. The CT specimen310 is coupled to a plate 314 and a trigger transducer 318 is placed onthe CT specimen 310. A data transducer 322 is disposed on the plate 314,desirably at 24 inches away. The trigger transducer 318 on the edge ofthe plate 314 is used to provide a trigger signal for the digitalvoltmeter such as the one shown in FIG. 7, voltmeter 22, when 0.3 mmPentil pencil lead breaks are made at different depths in the specimen310 shown by the parallel lines 328 on the edge of the specimen 310. Thepencil lead breaks are desirably made at each level of the parallellines 328. Another transducer 334 shown is desirably used to detect theAE data from the specimens 310 for each lead break. Signals in the plate314 are recorded by the data transducer 322, which is desirably a falseaperture transducer such as the transducer 10 of FIG. 1. The triggertransducer 318 is used so that velocity of sound for the different wavescan be measured and phase shift of the signals can be observed. Thetrigger transducer 318 is desirably a small 650 kHz transducer used toprovide the capability of making velocity measurements of the varioustypes of waves produced in the narrow plate 314.

The specimen transducer 334 desirably has an ⅛-inch aperture with thepiezoelectric crystal mass loaded. The aperture and size of the mass isdesirably adjusted so that the transducer 334 is equally sensitive toOOP and IP signals in the specimen 310. The out-of-plane (OOP) signalmay be generated by breaking the pencil lead on the top or bottomsurface. The transducers 318 and 334 are desirably attached to the CTspecimen 310 with hot glue.

The CT specimen 310 is coupled to the plate 314. Vaseline is desirablyused to couple the CT specimen 310 to the end of the ½×3×42-inch longsteel plate 314. The data transducer 322 is desirably coupled to theplate 314 with vaseline, desirably at a distance of 24 inches from theend of the plate 314. 0.3 mm pencil leads may be broken on the CTspecimen 310 at different depths signified by the horizontal lines 328on the specimen 310 as in the previous experiment, as shown in FIG. 8.

FIG. 9 shows the low-frequency data from the specimen transducer 344 onthe CT specimen 310 for an OOP signal (a), and IP signals from 25% (b),50% (c), and 75% (d) depth in the CT specimen 310. An OOP signal fromthe bottom surface of the specimen 310 is also shown (e). Thehigh-frequency signals change very little as a function of depth, andhence only the signal from the 50% (f) depth is shown.

In order to provide a calibration of structure in this matter, one needsto be assured that the data recorded is representative of what one wouldobtain on the structure alone. The CT specimen 310 is hence removed andthe trigger transducer 318 attached to the end of the plate 314 with hotglue. Pencil leads may be broken at the same percentage depth in theplate 314 as is used for the CT specimen 310 with the data transducer322 at 24 inches from the end of the plate 314.

As shown in FIG. 7, the trigger transducer 214 represents the triggertransducer 318 of FIG. 8 and starts the sweep of the digitaloscilloscope 220. The signal from the data transducer 322, representedin FIG. 7 as 210, is split into a high-frequency (HF) component and alow-frequency (LF) component. The high-frequency (HF) component ispassed through the 100 kHz high-pass filter 226, and the low-frequency(LF) component through the 20-60 kHz bandpass filter 228. Each componentis amplified by a variable gain amplifier 234 or 236 and then input intoa digital voltmeter 220. The digital oscilloscope 220 captures thetransient signals and passes them to the computer, and the waveforms arethen printed.

Referring to FIG. 8, the breaking of pencil leads at different depths inthe CT specimen 310 unexpectedly produces dramatic changes in signallevel of the low-frequency (LE) channel, even for such a thick structurewhich is substantially thicker than a thin plate. An OOP source resultsin a large low-frequency flexure-type wave being set up in the specimen310. Lead breaks at different depths on the edge of the specimen 310result in very little low-frequency signal at the point of symmetry atthe mid-plane of the specimen 310 with increasing amounts oflow-frequency signal at 25% and 75% for unsymmetrical input of the leadbreak. A phase shift is observed in the signals between depth levels onopposite sides from the mid-plane.

Experiment demonstrates that the response of the CT specimen 310 to leadbreaks at different depths can be “cloned” into the plate 314. This isillustrated by the large flexure wave signal generated by the lead breakon the top surface, compared to the very small flexure wave signalobserved from the lead break made at the mid-plane of the CT specimen310. The signals arriving at approximately 120 microseconds correlatewell with the extensional wave velocity of 200,000 in/sec (5,000 M/sec)in steel. The first large signal has an arrival time corresponding tothe shear velocity of approximately 130,000 in/sec (3,300 M/sec). Themeasured velocities of 200,000 in/sec and 131,000 in/sec correspond tothe extensional and shear velocity of sound in steel. The velocitycalculations may be made using the 10 microsecond markers from the 100kHz sine wave that may also be recorded with the data.

A calibration curve based on the ratio of the peak amplitude of thehigh-frequency signal to the peak amplitude of the low-frequency signalis shown in FIG. 10. One can observe from the data in FIG. 10 thatout-of-plane (OOP) sources for all three situations represented by FIG.10 have ratios of less than one. A filter can therefore be constructedfor data of this type by setting up the instrumentation and software toonly accept as valid signals those signals having a high frequency tolow frequency peak amplitude ratio greater than 1. Since extraneousnoise sources are primarily OOP sources, they can be eliminated from thedata set early by simple analog front end filtering as used in theseexperiments coupled with a simple software calculation and algorithm.

The methodology just described can also be used for field calibration ofa field structure. The use of a calibration specimen is particularlyadvantageous in the field because the structure generally does not havean accessible edge for breaking pencil leads to simulate crack growth.The portability and adaptability of the instrumentation and apparatus ofFIGS. 7 and 8 lend themselves to field testing.

It is very difficult to always put the same signal into the plate 314with a pencil lead break. One must be very careful to hold the pencilparallel to the plane of the plate 314 when breaking the leads. Anyforce perpendicular to the plane of the plate 314 will tend to induceflexure waves in the plate 314. Because a ratio of two components of thesame signal is computed, the ratio is not highly dependent on theabsolute amplitude of the signal.

Breaking pencil lead on the surface of structures or specimens isgenerally not a good simulation of AE signals one might expect fromcrack growth in metals or fiber breakage in composites. It is verydependent on the location where the lead is broken and the pencilorientation in respect to the plane of the structure or specimen. Themethod and apparatus of simulating acoustic emission signals produced bycrack growth with a structure are described in my patent, U.S. Pat. No.5,014,556, which may be used in place of the pencil-break technique forimproved performance, and is incorporated herein by reference.

2. Measurement

Once calibrated, the CT specimen or calibration block 310 is removed anda trigger transducer, such as transducer 318, is put in its place (notshown). Continuous monitoring of the crack proceeds and ratios arecalculated for all received signals, as well as time of flight betweenthe trigger transducer 318 and data transducer 322. The time of flightis used to construct a time-related filter. The ratio of each signal iscalculated and signals that come from OOP sources are rejected. If asignal passes the ratio test, the time of flight between triggertransducer 318 and data transducer 322 is observed. If the time does notfall within a predetermined window, the data is rejected as coming fromother cracks or extraneous sources. This forms a time-related filter orspatial filter for the signal. If the signal passes the time test, itsratio is compared to the calibration curve to estimate the depth of thecrack responsible for the signal. As the crack continues to grow, it isobserved whether or not the ratio is increasing or decreasing. Ifincreasing, the crack is less than half way through the plate; ifdecreasing, the crack has passed the mid-plane of the plate 314 (FIG.8). The trigger transducer 318 will allow the change in phase of the lowfrequency signal to be observed. This information can also be used todetermine crack depth information.

E. Conclusions

This invention presents a new technique for using acoustic emission datato eliminate extraneous noise sources from a data base, and measure thedepth of a growing crack in structures. This method can have tremendousimpact on use of the technology for monitoring of structures in thefield. Although the only information that can be measured is crackdepth, it is the most important parameter affecting structural integrityfor surface cracks. Some assumptions concerning the crack shape andphysical measurement of the crack length at the surface allows one todetermine the profile of the crack tip for a crack that has notpenetrated through the thickness.

The simulated response of the data transducer 322 of FIG. 8 to crackgrowth may be validated by physically loading the specimen 310 afterhydrogen charging to create crack growth, and correlating the AE datawith actual crack growth.

It will be understood that the above-described arrangements of apparatusand the methods therefrom are merely illustrative of applications of theprinciples of this invention and many other embodiments andmodifications may be made without departing from the spirit and scope ofthe invention as defined in the claims.

What is claimed is:
 1. A method for substantiaily eliminating extraneousnoise sources during nondestructive testing of a structure to detect thegrowth of a crack in the structure comprising: sensing acoustic emissionin said structure to produce an electrical signal, said signal includinga high frequency component whose magnitude is substantially determinedby the size of a crack or flaw in said structure and a low frequencycomponent whose magnitude is substantially determined by extraneousnoise; extracting said high frequency component and said low frequencycomponent from said electrical signal; and computing the ratio of theamplitude of said high frequency component to the amplitude of said lowfrequency component, whereby the extraneous noise is substantiallyeliminated to provide a signal indicative of the acoustic emissionsignal produced by growth of said crack or flaw in said structure.
 2. Amethod for substantially eliminating extraneous noise sources duringnondestructive testing by detecting an acoustic emission resulting froma change in the structure of the material under test comprising: sensingacoustic emission in the material to produce an electrical signal;splitting said signal into a high-frequency component and alow-frequency component; and computing the ratio of said high frequencysignal to said low frequency signal, to provide filtering tosubstantially only accept ratios indicative of said structural change,whereby the extraneous background noise is substantially separated outto provide a signal indicative of the acoustic emission signal producedsubstantially only by crack growth in the structure.
 3. The method fornondestructive testing of claim 2 wherein said structural change isgrowth of a crack in said structure.
 4. A method for eliminatingextraneous noise sources from the data produced during acoustic emissiontesting comprising: sensing acoustic emission in a structure to producean electrical signal; splitting said signal into a high-frequencycomponent and a low-frequency component; and computing the ratio of saidhigh-frequency component to said low frequency component and acceptingsubstantially only ratios indicative of crack growth.
 5. A method ofdetecting crack growth in a structure in the presence of extraneousnoise signals comprising: sensing acoustic emission signals in astructure, said signals including an inplane (IP) high-frequencycomponent signal due to crack growth and an out-of-plane (OOP) lowfrequency signal due to extraneous noise sources such as impact andfriction: determining a high-frequency peak amplitude of saidhigh-frequency component and a low-frequency peak amplitude of saidlow-frequency component; and computing the ratio of said high-frequencypeak amplitude to said low-frequency peak amplitude in order torecognize if the signal came from crack growth or extraneous noise. 6.The method of claim 5, wherein determining a high-frequency peakamplitude and low-frequency peak amplitude includes: splitting saidsignal into said high-frequency component and low-frequency components;amplifying said high-frequency component; filtering said high-frequencycomponent with a high-pass filter; measuring a high-frequency peakamplitude; amplifying said low-frequency component; filtering saidlow-frequency component with a bandpass filter; and measuring alow-frequency peak amplitude.
 7. The method of claim 6, wherein saidhigh-pass filter has a frequency range above 100 kH and said bandpassfilter has a frequency range of 20-80 KHz.
 8. The method of claim 6,wherein the ratio of the high frequency peak amplitude to low-frequencypeak amplitude for a particular plate thickness is computed for an OOPsimulated extraneous noise source by breaking a pencil lead on thesurface of the structure undergoing an acoustic emission test.